Develop Reference

<androidx.appcompat.widget.AppCompatEditText>

hello i want to use androidx library but it's not showing for me and i tried Refactor->Migrate to androidx and it didn't work too
can i get help please..
https://i.stack.imgur.com/xHNJ9.png

Make sure all those dependencies are added to your project. Then still face your problem go to the android studio file->invalid caches and restart.
https://i.stack.imgur.com/cXzGu.png

I want to use androidx library but it's not showing for me and I tried Refactor->Migrate to androidx and it didn't work too
Looking at below picture: https://i.stack.imgur.com/cXzGu.png
You can't use androidx for TextInputEditText because that widget is a property of Google material design. That means, the intellisense autocomplete dialog is showing you the exact and correct code. Check here and see the artifact mapping list of android support library to androidx or android jetpack
But before then, according to this post it reads:
With Android Studio 3.2 and higher, you can migrate an existing project to AndroidX by selecting
Refactor > Migrate to AndroidX from the menu bar.
The refactor command makes use of two flags. By default, both of them are set to true in your gradle.properties file:
android.useAndroidX=true
The above ☝️ will allow Android plugin to make use of the appropriate AndroidX library instead of Android Support Library.
android.enableJetifier=true
The above ☝️ Android plugin automatically migrates existing third-party libraries to use AndroidX by rewriting their binaries.
In conclusion, if above lines of codes are not in your gradle.properties file then you can add them and sync gradle
Note: The built-in Android Studio migration might not handle everything. Depending on your build configuration you may need to update your build scripts and Proguard mappings manually. For example, if you maintain your dependency configuration in a separate build file, use the mapping files mentioned below to review and update your dependencies to the corresponding AndroidX packages.

Can I use library that uses Androidx with support library project?

My project uses support library and library which i have added as dependency uses androidx.Can i build app without replacing my project's support library APIs with androidx?
I have already seen this answer but my question is opposite of that.

Using support libraries is not a recommended practice anymore, and you should migrate to androidx as soon as possible. Migration is easy, and Android Studio takes care of the heavy lifting for you.

Is the androidx.* necessary when I create a new project in Android Studio 3.4.2?

In my mind, androidx.* is optional for an Android project, I find that androidx.* is necessary when I update to Android Studio to 3.4.2.
See the Image. So androidx.* will be standard, right?
Image

AndroidX is a major improvement to the original Android Support Library.
Like the Support Library, AndroidX ships separately from the Android OS and provides backwards-compatibility across Android releases. androidX fully replaces the Support Library by providing feature parity and new libraries.
Here
If your app currently depends on the original Design Support Library, you can make use of the Refactor to androidX option provided by Android Studio. Doing so will update your app’s dependencies and code to use the newly packaged androidx.
New Design support library requires andriodX migration too! You should consider using androidX for your future projects! Here
Add the library to the in your build.gradle(app-level) dependencies section:
implementation 'com.google.android.material:material:version' Latest Version

you can still make app absolutely..androidx is just a extension of the android library itself...so if you completely know android libraries,you can make any app,same as what androidx can,as the name implies,its a "support library" so without using it you can still make full blown app...again its a support library to make your life not worrying of compatibility for your app,but if your really know android programming,your dont need androidx.infact making app in android(not androidx)is better because you dont need transitions in the future same to what happen to those who rely on android support libraries..

How to use the library on AndroidX?

I have written a library that supports AndroidX.
How can I use this library with migration if I don't want to migrate my project to AndroidX yet?

Many popular libraries that have supported AndroidX say something like "If you wish to use this library now, migrate to AndroidX or use old versions". So I don't think you can use an AndroidX library with an non-AndroidX project.
Support libraries and AndroidX have different package name space. In the future, new changes in the AndroidX libraries won't be available in the support libraries.
Migrate the project to AndroidX or downgrade the library to support library.
Update
As mlyko's comment mentioned, jetifier has a reverse mode:
https://developer.android.com/studio/command-line/jetifier#reverse_mode
In this mode, the utility converts AndroidX APIs to the support library equivalents, instead of the other way around. Reverse mode is useful, for example, if you are developing libraries that use AndroidX APIs, but also need to distribute versions that use the support library.
I haven't tried it but it seems promising.

You could try the jetifier-standalone in reverse mode, to provide build with support libraries. Therefore you'd have 2 releases of the library - androidx and support lib.

Difference between androidx and com.android.support

I was about to use a constraint-layout in my project when I noticed that there are two different dependencies that I can use:
com.android.support.constraint:constraint-layout
androidx.constraintlayout:constraintlayout
Is there a difference between these two or some recommendation on which is preferable?
EDIT
Google is stopping support for com.android.support and prompts users to migrate to the new androidx equivalent.
Note: With the release of Android 9.0 (API level 28) there is a new version of the support library called AndroidX which is part of Jetpack. The AndroidX library contains the existing support library and also includes the latest Jetpack components.
You can continue to use the support library. Historical artifacts (those versioned 27 and earlier, and packaged as android.support.*) will remain available on Google Maven. However, all new library development will occur in the AndroidX library.
We recommend using the AndroidX libraries in all new projects. You should also consider migrating existing projects to AndroidX as well.
Here is the official Migration guide and the corresponding library equivalents.

All the support libraries are dropping the v4 v7 v12 v13 etc tags and everything is refactored into the androidx packages.
They are essentially the same but for future reference androidx will be the library that we should use in our apps.
Android studio 3.2 canary that comes out this week (week of May 14, 2018) should have the tool that allows automatic refactoring to the androidx packages. There was an announcement about this at google i/o 2018.

One of the difference between AndroidX and Support libraries is that when you use Support libraries, all Support libraries must be the same version, but in AndroidX there is no such thing.
Other thing is that in Support library, most of the time when you need a component in your app, you have to add dependencies that have so many other thing that you really don't need. But in AndroidX you can add only the dependency that you need and no more.

This might help,
There are a few differences as follows:
With the current naming convention, it isn’t clear which packages are bundled with the Android operating system, and which are packaged with your application’s APK (Android Package Kit). To clear up this confusion, all the unbundled libraries will be moved to AndroidX’s androidx.* namespace, while the android.* package hierarchy will be reserved for packages that ship with the Android operating system.
For example: android.content.Intent; is Android OS dependent & androidx.fragment.app.Fragment; this is shipped with APK
Initially, the name of each package indicated the minimum API level supported by that package, for example support-v4. However, version 26.0.0 of the Support Library increased the minimum API to 14, so today many of the package names have nothing to do with the minimum supported API level. When support-v4 and the support-v7 packages both have a minimum API of 14, it’s easy to see why people get confused!. So now with AndroidX, there is no dependence on the API level.
This is just an extension to the 2nd point, another important change is that the AndroidX artifacts will update independently, so you’ll be able to update individual AndroidX libraries in your project, rather than having to change every dependency at once. Those frustrating “All com.android.support libraries must use the exact same version specification” messages should become a thing of the past!

As said above and in Migrating to AndroidX, you can easily migrate in AS:
With Android Studio 3.2 and higher, you can quickly migrate an
existing project to use AndroidX by selecting Refactor > Migrate to
AndroidX from the menu bar.
Concerning the difference (TLDR: no impairments, emphasis mine)
AndroidX maps the original support library API packages into the
androidx namespace. Only the package and Maven artifact names changed;
class, method, and field names did not change.